In general, a method of obtaining an oxide superconducting wire by heat-treating a wire formed by charging a metal tube with raw material powder of an oxide superconductor and thereafter wiredrawing and rolling the metal tube for sintering the raw material powder of the oxide superconductor is known as a method of manufacturing an oxide superconducting wire. However, the wire is blistered in the aforementioned heat treatment step for sintering, to disadvantageously reduce the superconductivity of the obtained oxide superconducting wire.
Japanese Patent Laying-Open No. 5-101723 proposes a method of manufacturing an oxide superconducting wire by heat-treating a metal tube charged with powder of an oxide superconductor or a flattened body thereof for sintering the powder of the oxide superconductor. The aforementioned gazette describes that a wire having excellent superconductivity is obtained according to this method by performing a pressure heat treatment.
More specifically, it is attempted to store a metal tube charged with powder of an oxide superconductor in a heat-resistant/pressure-resistant closed vessel for preventing blistering in sintering by increasing the internal pressure following temperature rise in the closed vessel. The aforementioned gazette describes that the current internal pressure can be obtained from a state equation of gas or the like, and an internal pressure of about 4 atm. can be obtained with a heating temperature of about 900° C., for example.
Japanese Patent No. 2592846 (Japanese Patent Laying-Open No. 1-30114) proposes a method of manufacturing an oxide superconducting conductor by holding a metal tube charged with oxide superconducting powder or the like in a high-pressure state at least either in a heat treatment or after the heat treatment. The aforementioned gazette describes that partial separation on the interface between the oxide superconductor and the metal tube caused in sintering can be prevented according to this method by setting the metal tube in the high-pressure state.
More specifically, the metal tube charged with the oxide superconducting powder can be press-fitted to a sintered body by holding the metal tube in a high-pressure state of 500 to 2000 kg/cm2 (about 50 to 200 MPa) at least either in the heat treatment or after the heat treatment. Thus, when the superconductor partially causes quenching, heat resulting from this quenching can be quickly removed. In addition, it is also possible to prevent deterioration of the superconductivity resulting from a separation part forming a stress concentration part causing distortion.
In Japanese Patent Laying-Open No. 5-101723, however, the internal pressure obtained following temperature rise in the closed vessel is about 4 atm (0.4 MPa). Thus, voids are formed between oxide superconducting crystals in sintering, to disadvantageously reduce the critical current density.
Further, the oxide superconducting wire cannot be sufficiently inhibited from blistering caused in sintering due to the internal pressure of about 4 atm (0.4 MPa), and hence the critical current density is also disadvantageously reduced.
In the method according to Japanese Patent No. 2592846, it is difficult to control the partial oxygen pressure in the heat treatment due to application of the excessively high pressure of 500 to 2000 kg/cm2 (about 50 MPa to 200 MPa), to reduce the critical current density.